Many years ago when I was studying ecology and plant physiology, I learned that different shades of green represent slightly different (differently modified) photosynthetic pigments, which are optimal for different ecological niches, depending on the amount and quality of light. For example, the mosses and some other plants adapted to grow in the forest have chlorophyll pigments that are optimized to get the most efficient light capture in a shade situation.
Maybe tomatoes are able to adapt their pigments for different light regimes.
On the other hand, when there's a lot of purpling on the leaf, it may alter the shade of green that we perceive too.
When I've put tomato plants outdoors here (much colder) some of them will completely lose their medium green tender greenhouse foliage in the first week or so. The new foliage is always tighter and a darker green, and is really rugged to the elements.
Don't know if the colour is from modified chlorophylls, other substances/pigments eg the purpling, or maybe due to slower growth, the pigment density is a variable too...???
Just some rambling thoughts.
